Forum: Lessons from a rainforest - Horticulturists should get out into the real world, says Debbie Macklin

If I had had to choose a place to bump into a horticulturist from Edinburgh’s
Royal Botanic Garden, I don’t think I could have picked a better one than
the lush rainforests of eastern Brunei. It was a novel place, too, for David
Mitchell, who had never before set foot outside the British Isles, even
though his job for the past five years has revolved around nurturing tropical
forest plants in Edinburgh’s hothouses.

Opportunities for horticulturists to witness the natural growing conditions
of their exotic species are all too rare, Mitchell told me. His short visit
to Brunei had convinced him that, if this situation were reversed, we might
see more exotic plants growing better in our botanical gardens, and an explosion
of ideas for presenting them to the public in more natural settings than
at present. More importantly, horticulturists have a role to play in the
conservation and breeding of rare plant species-a task which would surely
benefit from more field experience.

Mitchell was in Brunei thanks to the far-sightedness of George Argent,
a plant taxonomist from Edinburgh. Argent had invited him to help survey
and collect plant species for the Brunei Rainforest Project-a 14-month research
expedition organised by the Royal Geographical Society and the University
of Brunei.

So there he was, keen as mustard, tightening his belt by three holes
more than usual as the sweat flowed from his brow. When I caught up with
him, Mitchell was three weeks into his trip, but still marvelling at the
growing conditions which were beyond his expectations and preconceptions.

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‘This whole environment is so full of energy and life,’ he enthused.
‘Out here in the tropical sunshine and high humidity, with temperatures
in the high 20s, no wonder things grow so well.’

Typically, it seems, horticulturists dealing in tropical forest plants
must rely on books, David Attenborough programmes, feedback from field scientists
during coffee breaks-even Tarzan films-to build up an impression of the
growing conditions in a rainforest environment.

That’s simply not enough, said Mitchell. He had been doing his own bit
of environmental monitoring and was bowled over by the humidity levels in
different parts of the forest at different times of the day. Early morning,
down by the river’s edge, relative humidity was hitting 92 per cent. Back
in Edinburgh’s hothouses, plants native to such parts have to make do with
morning humidity levels of just 55 per cent.

‘Even employing various cultivation techniques, such as damping down
and spraying, we can’t get overall hot house humidity levels up much beyond
70 per cent’ said Mitchell. ‘Most plants will grow reasonably well at this
level, but for those species which don’t do so well, at least we could try
to provide a more humid microclimate, if that is what they are in need of.’

If humidity levels were significantly higher than Mitchell had supposed,
the opposite was true for the amount of light available to many forest plants.
Many of the Zingerbraceae, Gesneraceae and Ericaceae, which Argent and Mitchell
had made the focus of their attention in Brunei, were receiving little more
than a couple of hours of direct sunlight a day.

‘The plants here are getting an injection of sunlight for a proportion
of the day, but not for the whole day,’ said Mitchell. ‘And the day length
here is so balanced throughout the year. Back in Scotland, we can have very
long days during the summer-up to 18 hours of daylight-and in the winter
as little as 6 hours. The plants in Edinburgh have to adapt to this very
difficult light regime.’

Another striking feature of the rainforest which Mitchell had not fully
appreciated before seeing it for real was the carpet of leaf litter lying
on the forest floor. It made him think twice about the extensive use of
peat as a growing medium by horticulturists.

Environmentalists don’t like the current exploitation of peat in the
gardening world because it is devastating the distinct flora of peat bogs.
Having seen the rich layer of leaf litter that feeds the forest, Mitchell
now maintains that peat is hardly the most appropriate growing base for
landscaped hothouses. ‘Peat may still have a role to play in the pot plant
industry, but in other circumstances-for example, the display borders at
Edinburgh-I’m convinced that we need to go back to traditional methods of
using leaf mould and organic matter.’

Soil temperatures, too, took Mitchell by surprise. Glasshouse soil temperatures
in Edinburgh are about 15 °C. In Brunei, Mitchell found them to be consistently
above 24 °C-another factor which could have a significant impact on
the growth patterns of some plants in the botanic gardens. ‘We may not need
to change soil temperatures for a whole glasshouse,’ said Mitchell. ‘We
may just need to change them for a single bed. Maybe some of the gingers
that are having difficulty flowering in Edinburgh should be put in a separate
bed with a cable running through to heat the soil to 24 °C’

When Mitchell left Brunei, he took with him a bundle of carefully packaged
living plants (all fully authorised for export, of course). If they survived
the journey, and quarantine in Edinburgh, their chances of blooming in full
glory will no doubt be enhanced when Mitchell translates his experience
of the forest into practice. He has been charged with designing the exhibition
hall for the World Orchid Congress, due to be held in Glasgow in 1993. It
is bound to reflect some of the most memorable features of Brunei’s forests.

Gardeners, scientists and conservationists alike need the skills of
horticulturists: skills which would surely be enhanced if more had opportunities,
like Mitchell, to seek ideas and inspiration from the natural world.